The present invention relates in general to pressure vessels and in particular to a new and useful pressure vessel having a substantially toroidal shape with a cylindrical outer contour, the vessel having meridional and equatorial windings.
As a rule, pressure vessels of compound material, i.e. made of resin-impregnated glass fibers, carbon fibers, or the like, comprise a gastight or liquid tight inner tank which is armored or reinforced with a back-up winding of fibers, threads, or wires. This back-up winding removes lead from the inner tank, so that the total pressure forces acting on the vessel is taken up partly by the inner tank and partly by the back-up winding. In the following, such pressure vessels are termed optimal, if their contour has a meridional curvature at which the constant tangential and meridional stresses in the wall of the inner tank are equalized and, at the same time, the tensions of the elements in the back-up winding are kept constant. The equality of tangential and meridional stresses permits a full utilization of the load-carrying capacity of the employed material of the inner tank.
To satisfy this stress condition in a non-wrapped inner vessel, spherical vessels and special toroidal zones having a non-constant curvature, must be provided. Such optimal vessels have a weight amounting to one and a half the weight of the maximum storable energy, i.e. the pressure multiplied by the volume and divided by the tearing length of the material used.
The tearing or breaking lengths of wires, threads, and fibers and particularly large. Thus the tearing or breaking lengths of structural elements for taking up stresses in a single direction, namely their longitudinal direction, are large. In producing a vessel by wrapping a core, optimal lightweight pressure vessels are obtained if the curvature of the meridian is chosen so as to have the effect of equalizing the stresses of the threads, fibers, or wires in geodesic extension at any location, so that the load carrying capacity of the filamentous material is fully utilized.
Vessel shapes satisfying this condition are the cylinder, and tubular vessels having a periodically varied diameter, as well as toroidal zones having a non-constant meridian curvature. The weight of such optimal lightweight wound vessels amounts to triple the maximum storable energy, referred to the breaking length of the material used.
In the case of a wound construction, an additional weight proportion, of course, goes to the binder used, such as a resin, for binding the threads together. This weight proportion is particularly high if the filament is reversed in direction near some polar zone (criss-cross winding), and particularly low if the filaments extend in parallel (such as if a winding perpendicular to the meridian is applied to a cylinder). On toroidal vessels, a criss-cross winding is not used.
While applying the winding or wrapping technique, two principal difficulties are to be overcome:
1. With any kind of winding, polar zones of the vessel must be taken into account. If the materials (simply termed filaments in the following) are wound in the meridian direction, all filaments passing the equator cross at the pole. Therefore, special additional structural elements, such as connections and covers, are needed for the polar zones. This problem does not arise with toroidal vessels.
2. Vessels which are only wound and do not contain any inner tank, are liquid tight and gastight only under relatively low pressures. For this reason, the vessel may be wound on a core which is then washed out and replaced by a rubber bag inflated through a filling connection. This, however, is not possible with toroidal pressure vessels. Another possibility is to wind the filaments on an inner tank which remains in the pressure vessel and forms the winding core and then serves as the sealing wall.
While using as inner tank as supporting core for the back-up winding, care must be taken to provide the same absolute extensions for the material of both the inner tank and the wound filaments, i.e. both these materials must have the same modulus of elasticity, that is, in both materials, the stress and the modulus of elasticity must have the same proportion. Consequently, in a regular case, only the breaking length of the filaments or the breaking length of the inner tank can fully be utilized. If it is desired, for example, to fully utilize the high strength of carbon fibers, it is not possible to utilize, at the same time, the high breaking length of the material used for the inner tank, such as titanium. For this reason, it is advisable to take up the maximum possible load proportion through the winding, and a corresponding only very small proportion through the inner tank.
Now, an optimal wrapping of optimal vessels may be obtained by providing a proper meridional curvature of the vessel ensuring that the back-up pressure exerted by the winding on the loaded inner vessel is not constant, independently of the radius of the vessel, but effects a certain non-uniform pressure distribution over the vessel radius, such that the chosen shape of the vessel is at the same time the best or optimal one for the non-uniformly loaded inner tank and optimal for the non-uniformly loaded outer winding. This combination of an inner tank and a back-up winding would have an entirely uniform extension in any direction and at any point. Both the material of the inner tank and the material of the winding are fully stressed to an extent to which the winding and the inner tank are capable of equal extension.
With a high load proportion taken up by the winding and a very small load proportion absorbed by the inner tank, the mentioned design leads to toroidal zones of non-constant meridional curvature, yet not to meridians closed in themselves. The design might also be applied to a tank in the form of a closed torus. The disadvantage of this tank which is optimal in itself is that the load distribution between the inner tank and the outer back-up winding sets limits to the design. For example, it is not possible to utilize the very high breaking length of carbon fibers beyond a certain degree, since then a substantial portion of the load is necessarily taken up by the inner tank.